the FOV in the game is sadly really narrow even on max settings.
the camera is way too close too.
I also think the way Leon moves, and the way the devs programmed the camera to move can add to feeling motion sick for some people.
the issue IMO being that the camera doesn't move smoothly.
in most games, as you walk straight forwards, the camera and character model move smoothly with a consistent speed... let's say 2km/h.
in REmake 4 however leon and the camera move "more realistically". Even if you move perfectly straight forward, Leon will take a step and then slow down a bit as he moves his foot forward to take the next step, this gets worse if he is wounded or was recently hit.
the camera ALSO moves at those inconsistent speeds. meaning if Leon slows down for half a second, so does the camera, and soon after when he speeds up for a second again, so will the camera too.
so the camera will go from (again these are made up speeds for the sake of demonstration) 2km/h to 1km/h and back to 2km/h at semi-regular intervals, which is also changed slightly whenever you are even minimal change the movement direction of Leon, which is why I said semi-regular intervals.
because the way he moves his feet, and therefore the speed at which he moves, and the interval between him speeding up and down, changes every time you turn a corner, get hit, sidestep, etc.
for me this doesn't make me motion sick, but it adds to the sluggish feeling of the game as a whole, and why IMO the original game feels way better to play.
but I can see that maybe adding to making people motion sick.
if you add this irregular camara speed with fast mouse movement, I could totally see it being an issue for some.
add to that how close the camera is and how narrow the FOV is and you got a bunch of causes that can all add up if you're sensitive to them.